Municipal waste water and industrial waste water e.g. from food industry, contain organic material and are also rich in nitrogen and phosphorous compounds, and thus purification of such waste water is necessary before the purified water is discharged to natural recipients like lakes, rivers and the sea.
Biological reactors are used for breakdown of organic and inorganic substrates in municipal and industrial waste water. Commercially there are two main types of biological reactors i) activated sludge reactors and ii) biofilm reactors. In activated sludge reactors the microorganisms are suspended in the waste water. In a biofilm reactor the microorganisms grow on carrier media. The carrier media can either be stationary e.g. surfaces of large drums or the reactor walls or free flowing smaller carrier elements.
Activated sludge reactors allow a continuous process; however such reactors are very area/volume consuming, and this is also the case for the final sludge separation by sedimentation. A portion of the sludge is recycled into the reactor or inlet stream of the reactor to maintain the microbiological culture.
Moving bed biofilm reactors are operated continuously or in batches. Enhanced biological phosphorous removal can only be applied in such a system if it is run in batches, i.e. the waste water is let into the reactor for treatment and the treated water is discharged before the reactor can be refilled with waste water. This implies that for each volume treated there is a time consuming filling and draining sequence. The method is thus laborious and will normally demand that series of reactors are operated in parallel.
Separation of sludge and water after the biological process is easier for a biofilm process than an activated sludge process. A biofilm process needs a smaller reactor volume than an activated sludge process, and also has no recycling of sludge demanding process control and energy. Since there is no recycling from the separation step in the biofilm process, the performance of the separation does not affect the biological process.
WO 2010/140898 A1 relates to a method and apparatus for biological purification of water in a reactor with one or more inlet and outlet zones where water and substrate come into contact with carrier elements for a biofilm.
US 2008/0053897 A1 disclose a liquid-solid fluidized bed waste water treatment system for simultaneous carbon, nitrogen and phosphorous removal. The system incorporates the fixed-film biological fluidized bed technology with the biological nutrient removal in a twin liquid-solid fluidized bed, which has achieved the simultaneous elimination of organic carbon, nitrogen and phosphorous.
Other publications concerned with water purification/treatment JPh 07163994 A, JP h 07163995 A, DE 19501260 C1 and Li, M., Nakhla, G., Zhu J., “Simultaneous carbon and nitrogen removal with enhanced bioparticle circulation in a circulating fluidized bed biofilm reactor”. Chem. Eng. Jour. 181-182 (2012) 35-44.